Loading of an AC DB against selection regulations?

[Pete-JohnsonsPSG]

Helloo guys,

First post, so sorry if this is dreary! I'll try keep it simple...

I have a client with a distribution circuit supplying AC. installation is supplied via 160A MCCB, 70mm 5 core XLPE, 250 Amp rated Distribution board.. so far so good.

distribution board is loaded with 18 MCB's, seven are 32 AMP, nine are 25 AMP. Maximum loading to the circuit according to manufacturer is around 80% (21 Amp on plate of AC condenser). For sake of simplification I will say the max loading is 330 AMP, thats Maximum, assuming all 18 external condensers are trying to heat.
Now for the complex bit; what rating should my distribution circuit be? Do we apply any diversity when designing to BS761 when its an AC installation? there's nothing in the regs for AC. I have stated that the In of the distribution device should not be less than the Ib (design load) of the installation, and that would mean we should have a 400A distribution circuit (2x250AMP rated DB's fed from two separate 200AMP 70mm distribution circuits, sp[lit the AC final circuits in two)
But the client is throwing it back at me, saying the installation has been fine for three years so it is fine for continued use. Except the reason we are involved is the busbar feeding the MCCB burnt right through over the holidays, and 1 year ago the 250 Amp DB burnt out and was replaced like for like.

I'm after some healthy debate on what is correct for this installation. Obviously its protected as the MCCB is less than the rating of all the equipment, but prolonged is obviously creating issues

thanks for listening

Pete

 

[davesparks]

As its an existing installation you can put a data logger on it for a couple of weeks to record the average and maximum demand. Much easier to just measure these things than trying to work out the diversity etc.

What caused the busbar to be burned and what caused the DB be be burned?
Normally that sort of thins is caused by loose/high resistance connections.
Bearing in mind a 160A MCCB will protect the 250A DB from overload.

 

[Pete-JohnsonsPSG]

Hi

Unclear what the cause was. was not involved with the DB burn out and the MCCB burnt out caused so much damage we couldn’t tell if the terminal was loose where the busbar connected to the MCCB, the burnt out was about two inch from the terminal though, and only one phase, does scream loose connection.

I am thinking that generally the load stays at around 160 amp, as that would be the fan operating current without any heat and it’s a new build so it’s well insulated. It would trip, and it’s well below the rating of the distribution circuits, so risk is low.

But regulations? Is the potential for a problem Enough for it to be non compliant. I’m doubting whether they need to do anything at all given it will just trip, and doesn’t unless it’s really cold

Good idea with the logger tho, it will help identify any risk there may be

Pete

As its an existing installation you can put a data logger on it for a couple of weeks to record the average and maximum demand. Much easier to just measure these things than trying to work out the diversity etc.

What caused the busbar to be burned and what caused the DB be be burned?
Normally that sort of thins is caused by loose/high resistance connections.
Bearing in mind a 160A MCCB will protect the 250A DB from overload.

 

[Electro-tech]

Do you know what the overcurrent value of the 160A MCCB was set to, as this may be the downfall of the problems, thinking let's load the MCCB up more to stop tripping. Unfortunately, as you say no Diversity for AC in BS7671 so this leaves it to the engineer to decide the design.. I did come across a foreign revision of variable load consumption studies ''Based on the experiments mentioned in the above, the diversity factor for the air conditioner worked out to be 0.4 (i.e. 1.2kWh / 2.984kWh). However, it was recognized that the experiment was conducted in an office room within an airconditioned office environment. The heat gained and cooling energy leakages would be much less than in a residential environment. It was recommended that the diversity factor for split-unit air conditioners be based on an industry-practice value of 0.6. '' As Davesparks suggests data logger your best option.

 

[Pete-JohnsonsPSG]

Do you know what the overcurrent value of the 160A MCCB was set to, as this may be the downfall of the problems, thinking let's load the MCCB up more to stop tripping. Unfortunately, as you say no Diversity for AC in BS7671 so this leaves it to the engineer to decide the design.. I did come across a foreign revision of variable load consumption studies ''Based on the experiments mentioned in the above, the diversity factor for the air conditioner worked out to be 0.4 (i.e. 1.2kWh / 2.984kWh). However, it was recognized that the experiment was conducted in an office room within an airconditioned office environment. The heat gained and cooling energy leakages would be much less than in a residential environment. It was recommended that the diversity factor for split-unit air conditioners be based on an industry-practice value of 0.6. '' As Davesparks suggests data logger your best option.

Hey,

Mccb is set to 1 so would have been rated at the 160 amp. Which is considerably less than the capability of both the cable and the distribution board attached to it. I was wondering if consistent over load could have caused the burn outs over a matter of years.

My real concern is over long term damage due to poor design and regulation non compliance. Or maybe I’m being dramatic! It just seems to me the installation should be designed to meet the demand attached to it. Right now when it gets really cold it trips, but there isn’t, I don’t suppose any risk if the mccb is rated less than the capacity of the distribution